Method for regulating driving stabililty

ABSTRACT

The invention relates to a method for controlling the driving stability of a vehicle, wherein it is determined when a stable or unstable driving behavior prevails whether a tendency to a subsequent unstable driving behavior exists as a result of a highly dynamic inward swerve. In order to provide a method for controlling driving stability, which allows a reaction to predicted unstable driving situations by means of an intervention diminishing or avoiding critical driving situations, the steering force and/or the steering angle of a steering handle is corrected in this case in such a manner that when the steering handle is operated, the driver is assisted in the direction of an understeering vehicle course.

The present invention relates to a method for controlling the drivingstability of a vehicle according to the preamble of claim 1.

Abrupt steering and countersteering actions during e.g. obstacleavoidance maneuvers, lane changes and similar maneuvers can causeinstabilities of the vehicle at high coefficients of friction. There isan increased risk of rollover in vehicles with a high center of gravity.

A great number of driving stability control systems for automaticallycounteracting these vehicle instabilities have become known in the art.The term ‘driving stability control’ covers five principles ofinfluencing the driving behavior of a vehicle by means of predefinablepressures or brake forces in or at individual wheel brakes and by meansof intervention into the engine management of the driving engine. Thesesystems concern brake slip control (ABS) intended to prevent the lockingof individual wheels during a braking operation, traction slip control(TCS) preventing the spinning of the driven wheels, electronic brakeforce distribution (EBD) regulating the ratio of the brake forcesbetween front and rear axles of the vehicle, anti-rollover braking (ARB)preventing rollover of the vehicle about its longitudinal axis, and yawtorque control (ESP) safeguarding stable driving conditions duringyawing of the vehicle about its vertical axis.

Hence, the term ‘vehicle’ in this context refers to a motor vehicle withfour wheels equipped with a hydraulic, electro-hydraulic, orelectromechanical brake system. In the hydraulic brake system, thedriver can develop brake pressure by means of a pedal-operated mastercylinder, while the electro-hydraulic and electromechanical brakesystems build up brake force depending on the detected braking requestof the driver. Reference is made to a hydraulic brake system in thefollowing. Each wheel comprises a brake having one inlet valve and oneoutlet valve associated therewith. The wheel brakes are connected to themaster cylinder by way of the inlet valves, while the outlet valves leadto a non-pressurized reservoir or low-pressure accumulator. There isstill provision of an auxiliary pressure source that is able to build uppressure in the wheel brakes also irrespective of the position of thebrake pedal. The inlet and outlet valves are electromagneticallyoperable for pressure control in the wheel brakes.

To detect conditions related to driving dynamics, there is provision offour rotational speed sensors, one per wheel, a yaw rate sensor, alateral acceleration sensor, and at least one pressure sensor for thebrake pressure generated by the brake pedal. Instead of the pressuresensor, a pedal travel sensor or pedal force sensor may also be used ifthe auxiliary pressure source is arranged in such a way that it isimpossible to distinguish the brake pressure built up by the driver fromthe pressure of the auxiliary pressure source.

In driving stability control, the driving behavior of a vehicle isinfluenced in such a manner that the driver can master it better incritical situations. A critical situation herein implies an unstabledriving condition in which the vehicle does not follow thespecifications of the driver in the extreme case. Thus, in situations ofthis type, the function of driving stability control consists inimparting to the vehicle the vehicle performance desired by the driverwithin the physical limits.

While the longitudinal slip of the tires on the roadway is in first linesignificant for brake slip control, traction slip control, andelectronic brake force distribution, further quantities such as the yawrate and the sideslip angle gradient are included for consideration inyaw torque control (YTC). Anti rollover systems typically evaluatelateral acceleration quantities or roll rates (DE 196 32 943 A1).

It would be desirable to generally avoid unstable driving situations,which the driver frequently cannot master, so that critical drivingsituations cannot occur at all.

WO 02/36401 A1 discloses a method for predicting the behavior of a motorvehicle, which in dependence on the time variation of the steering anglevelocity determines during a stable driving behavior whether there is atendency to a subsequent unstable driving behavior. In the affirmative,braking pre-intervention will be executed already during the stabledriving behavior. Further, a method is described which takes place in anextreme countersteering maneuver during cornering at a high lateralacceleration and wherein the lateral acceleration is also taken intoaccount apart from the time variation of the steering angle velocity.Roll motions (rebound and compress) are produced by the high lateraldynamics of the vehicle body in extreme countersteering maneuvers duringcornering at a high lateral acceleration. The vehicle becomes unstable,involving the risk of rollover about its longitudinal axis.

Driving maneuvers of alternating dynamics such as a lane changerepresent a special challenge in terms of driving dynamics becausevarious forces must be accommodated (inertia force of the slidingvehicle, centrifugal force of the vehicle on a circular path), on theone hand, and because the wheel conditions (coefficient of frictionbetween tires and roadway, braking operation, reaction time of thedriver) based on which the driving maneuver can take place are variablein time, on the other hand. The unstable driving behavior of the vehicleduring driving maneuvers of alternating dynamics can be triggered orsupported by quick steering movements performed by the driver as areaction to an expected or an actual unstable driving behavior.Therefore, it would be desirable to provide the driver with all possibleaids being of assistance in critical driving situations.

An object of the invention is to disclose a method for controlling thedriving stability, which allows a reaction to predicted unstable drivingsituations by means of an intervention that diminishes or avoidscritical driving situations.

According to the invention, this object is achieved in that in this casethe steering force and/or the steering angle of a steering handle iscorrected in such a manner that when the steering handle is operated,the driver is assisted in the direction of an understeering vehiclecourse.

The unstable driving situation can be predicted based on a stable orunstable driving situation. The method makes use of the correction ofthe steering force and/or the steering angle of a vehicle with asteering handle such as a steering wheel, and the turning of said wheelproduced by the vehicle operator is converted by a steering mechanisminto a tilting movement of the steerable wheels of the vehicle. As thisoccurs, a control is performed in which the steering angle produced bythe driver is monitored to check whether the steering movement of thesteering handle causes an unstable driving behavior. If an unstabledriving behavior is produced by the steering movement or can developtherefrom, the change or modification of the steering force will inducethe driver to steer properly. The extent of the servo assistance can bereduced when the driver performs steering movements that lead to anunstable driving situation or augment an unstable driving situation whenthe vehicle is equipped with a servo steering system. In a vehicle whichis equipped with an auxiliary-power or servo steering system, e.g. anelectrically operable steering system, the steering force can be changedon command of electric actuating signals in such a way that themagnitude of the auxiliary force is increased or reduced, for example.Also, the steering angle can be changed in addition to prevent anunstable driving situation.

It is favorable that a pattern is determined in the initial phase ofsteering or during an inward swerve, respectively, by way of theamplitude and the frequency and, as the case may be, a zero passage ofthe steering angle with a substantially undiminished steering anglevelocity, said pattern being used to predict an unstable drivingbehavior.

When an imminent unstable driving behavior or an expected condition ispredicted on the basis of the current driving condition, the steeringforce and/or the steering angle of the steering handle is modified, e.g.raised or lowered and/or increased or decreased by means of a controlanticipating the predicted condition.

It is advantageous that the steering force and/or the steering angle ismodified in dependence on a performance graph of a steering powerassistance (steering ratio) taking the speed of the vehicle intoaccount.

It is favorably arranged that the performance graph is adjusted in thedirection of a speed higher than the actual speed of the vehicle.

Further, it can be expedient that the modification of the steering forceand/or the steering angle is adjusted in dependence on the period oftime until the unstable driving behavior sets in.

In addition, it can be suitable that the modification of the steeringforce and/or the steering angle is adjusted in dependence on acharacteristic curve.

To determine driving situations, it is favorable that the magnitude ofthe steering force and/or the steering angle is adjusted in dependenceon a track model. Advantageously, a maximum track divergence from thevehicle track desired by the driver is predefined in the track model,the desired track is compared to the maximum track divergence, and whenthe track divergence falls below the maximum value thereof, themagnitude of the steering force and/or the steering angle is reduced tosuch effect that the correction of the steering force or the steeringangle is withdrawn.

Vehicles can quickly tilt about their longitudinal axis in drivingmaneuvers of alternating dynamics. These rollover-critical situations indriving maneuvers of alternating dynamics can be caused bycorrespondingly quick steering movements, which the driver performs indriving situations that generally necessitate a quick reaction to thedriving behavior of the vehicle. To reduce the roll excitation or rollfrequency of the vehicle developing as a result thereof and causingrollover, a pattern detection of the rollover-critical steering anglesignal or steering angle speed signal is provided.

When a critical pattern in terms of frequency and amplitude is detectedin a lane change during the first inward swerve and, in addition, a zeropassage of the steering angle with a correspondingly undiminishedsteering angle speed is possibly determined, and when this pattern leadsto conclude a continuation of the maneuver, the characteristics ofsteering (steering force and/or steering angle) is changed. This changeis preferred in the direction of heavy steering because an increase ofthe steering angle (due to softer steering; higher steering assistance)in this situation does not have such a great influence on the drivingsituation because the vehicle is already at the driving-dynamics limit.

The steering characteristics can be influenced by a change-over of thespeed performance graph (a speed is assumed that is higher than thespeed at which the vehicle is actually riding). This change-over can beset depending on how near the critical situation is, or it can beapproached following a characteristic curve. The extent of the changecan be predefined according to a track model by way of a maximum trackdivergence from the track actually chosen by the driver, at which trackdivergence the influencing action is reduced again, as the case may be.This influencing action can be used both in servo steering systems(parameter steering operations) and in steer-by-wire systems.

1-11. (canceled)
 12. Method for controlling the driving stability of avehicle wherein it is determined during a stable or unstable drivingbehavior whether a tendency to a subsequent unstable driving behaviorexists as a result of a highly dynamic inward swerve, wherein thesteering force and/or the steering angle of a steering handle in thiscase is corrected in such a manner that when the steering handle isoperated, the driver is assisted in the direction of an understeeringvehicle course.
 13. Method as claimed in claim 12, wherein a pattern isdetermined in the initial phase of steering or during an inward swerve,respectively, by way of the amplitude and the frequency and, as the casemay be, a zero passage of the steering angle with a substantiallyunreduced steering angle velocity, said pattern being used to predict anunstable driving behavior.
 14. Method as claimed in claim 12, whereinthe steering force of the steering handle and/or the steering angle ismodified, e.g. raised or lowered and/or increased or decreased. 15.Method as claimed in claim 12, wherein the steering force and/or thesteering angle is modified in dependence on a performance graph of asteering power assistance (steering ratio) taking the speed of thevehicle into account.
 16. Method as claimed in claim 14, wherein thesteering force and/or the steering angle is modified in dependence on aperformance graph of a steering power assistance (steering ratio) takingthe speed of the vehicle into account.
 17. Method as claimed in claim15, wherein the performance graph is adjusted in the direction of aspeed higher than the actual speed of the vehicle.
 18. Method as claimedin claim 16, wherein the performance graph is adjusted in the directionof a speed higher than the actual speed of the vehicle.
 19. Method asclaimed in claim 12, wherein the modification of the steering forceand/or the steering angle is adjusted in dependence on the period oftime until the unstable driving behavior sets in.
 20. Method as claimedin claim 12, wherein the modification of the steering force and/or thesteering angle is adjusted in dependence on a characteristic curve. 21.Method as claimed in claim 12, wherein the magnitude of the steeringforce and/or the steering angle is adjusted in dependence on a trackmodel.
 22. Method as claimed in claim 21, wherein a maximum trackdivergence from the track desired by the driver is predefined in thetrack model, in that the desired track is compared to the maximum trackdivergence, and in that when the track divergence falls below themaximum value thereof, the magnitude of the steering force and/or thesteering angle is reduced to such effect that the correction of thesteering force and/or the steering angle is withdrawn.
 23. Electricallyoperable steering system for controlling the driving stability of avehicle, wherein it is determined during a stable or unstable drivingbehavior whether a tendency to a subsequent unstable driving behaviorexists as a result of a highly dynamic inward swerve, wherein thesteering force and/or the steering angle of a steering handle in thiscase is corrected in such a manner that when the steering handle isoperated, the driver is assisted in the direction of an understeeringvehicle course.
 24. Servo steering system for controlling the drivingstability of a vehicle, wherein it is determined during a stable orunstable driving behavior whether a tendency to a subsequent unstabledriving behavior exists as a result of a highly dynamic inward swerve,wherein the steering force and/or the steering angle of a steeringhandle in this case is corrected in such a manner that when the steeringhandle is operated, the driver is assisted in the direction of anundersteering vehicle course.